The present invention relates generally to the field of laser beam scanning systems, and more particularly to micro-electro-mechanical systems (MEMS) for laser beam scanning. Miniature laser beam scanning systems are important for applications such as barcode scanning, machine vision and, most importantly, xerographic printing. The use of MEMS to replace standard raster output scanning (ROS) in xerographic print engines allows simplification of printing systems by eliminating macroscopic mechanical components and replacing them with large arrays of scanning elements. Advanced computation and control algorithms are used in managing the large arrays of scanning elements. Such MEMS based printing systems are entirely solid state, reducing complexity, and allowing increased functionality, including compensation of errors or failures in the scanner elements.
An important step in constructing solid state scanning systems is integration of the semiconductor light emitter directly with MEMS actuators to gain the desired optical system simplification. Integrated scanners, which have lasers and scanning mirrors in the same structure, have been demonstrated using manual placement of laser chips onto MEMS wafers with micromachined alignment parts and adhesives by L. Y. Lin et al in Applied Physics Letters, 66, p. 2946, 1995 and by M. J. Daneman et al in Photonics Technology Letters, 8(3), p. 396, 1996. However, current techniques do not allow for wafer-scale integration of the light-emitter and MEMS device.
In accordance with the present invention a laser beam scanner consisting of a single crystal silicon deflection mirror and a torsional mirror is integrated with a laser diode in the same structure. Details of creating a torsional mirror and actuating it magnetically or electrostatically are detailed in U.S. Pat. No. 5,629,790 by Neukermans and Slater which is incorporated herein by reference in its entirety.
Using solder bump bonding methods, completed and tested laser diodes are bonded to a glass or a silicon carrier substrate. The carrier substrate is aligned and bonded to a Si or SOI wafer containing the MEMS layers. Bonding of the lasers to a carrier substrate completely partitions the bonding process from the MEMS. This complete partition eliminates possible conflicts between the conditions needed for solder bump bonding, such as the use of solder flux, and preserves the integrity of the MEMS layers.
The substrates are heated in a non-oxidizing environment to join the two substrates. High surface tension of the solder aligns the wettable metal bonding pads on each substrate with each other. The ability of the reflowed solder to self-align the substrates because of surface tension simplifies assembly.
The use of the SCS layer of a SOI wafer, rather than a polysilicon film provides for the introduction of very flat and smooth mirrors and high reliability torsion bars. The device is scalable to arrays of lasers and scanning mirrors.
Integration of the scanner and light source eliminates the need for external, manual alignment of light sources and scanning mirrors. Simplified post-processing steps such as interconnect metallization can be realized because the use of an etched recess results in nearly planar surfaces. In addition, pick and place technologies used for multi-chip module assembly can be adapted for wafer scale assembly and bonding of light sources to the carrier substrate.
Thus, the present invention allows the integration of lasers, electrical interconnects, and electrodes on a single glass or Si wafer for actuation of MEMS devices. The glass or Si wafer is aligned and bonded to the MEMS wafer, forming an integrated, three dimensional structure.